text: Ligands which cause a large splitting Δ of the d-orbitals are referred to as strong-field ligands, such as CN and CO from the spectrochemical series. In complexes with these ligands, it is unfavourable to put electrons into the high energy orbitals. Therefore, the lower energy orbitals are completely filled before population of the upper sets starts according to the Aufbau principle. Complexes such as this are called "low spin". For example, NO is a strong-field ligand and produces a large Δ. The octahedral ion [Fe(NO)], which has 5 d-electrons, would have the octahedral splitting diagram shown at right with all five electrons in the t level. This low spin state therefore does not follow Hund's rule.
  Conversely, ligands (like I and Br) which cause a small splitting Δ of the d-orbitals are referred to as weak-field ligands. In this case, it is easier to put electrons into the higher energy set of orbitals than it is to put two into the same low-energy orbital, because two electrons in the same orbital repel each other. So, one electron is put into each of the five d-orbitals in accord with Hunds rule, and "high spin" complexes are formed before any pairing occurs. For example, Br is a weak-field ligand and produces a small Δ. So, the ion [FeBr], again with five d'-electrons, would have an octahedral splitting diagram where all five orbitals are singly occupied.
  In order for low spin splitting to occur, the energy cost of placing an electron into an already singly occupied orbital must be less than the cost of placing the additional electron into an e orbital at an energy cost of Δ. As noted above, e refers to the
  d-y in octahedral complexes. If the energy required to pair two electrons is greater than Δ, the energy cost of placing an electron in an e, high spin splitting occurs.
  The crystal field splitting energy for tetrahedral metal complexes (four ligands) is referred to as Δ, and is roughly equal to 4/9Δ (for the same metal and same ligands). Therefore, the energy required to pair two electrons is typically higher than the energy required for placing electrons in the higher energy orbitals. Thus, tetrahedral complexes are usually high-spin.
  The use of these splitting diagrams can aid in the prediction of magnetic properties of co-ordination compounds. A compound that has unpaired electrons in its splitting diagram will be paramagnetic and will be attracted by magnetic fields, while a compound that lacks unpaired electrons in its splitting diagram will be diamagnetic and will be weakly repelled by a magnetic field.